can: isotp: set default value for N_As to 50 micro seconds

The N_As value describes the time a CAN frame needs on the wire when
transmitted by the CAN controller. Even very short CAN FD frames need
arround 100 usecs (bitrate 1Mbit/s, data bitrate 8Mbit/s).

Having N_As to be zero (the former default) leads to 'no CAN frame
separation' when STmin is set to zero by the receiving node. This 'burst
mode' should not be enabled by default as it could potentially dump a high
number of CAN frames into the netdev queue from the soft hrtimer context.
This does not affect the system stability but is just not nice and
cooperative.

With this N_As/frame_txtime value the 'burst mode' is disabled by default.

As user space applications usually do not set the frame_txtime element
of struct can_isotp_options the new in-kernel default is very likely
overwritten with zero when the sockopt() CAN_ISOTP_OPTS is invoked.
To make sure that a N_As value of zero is only set intentional the
value '0' is now interpreted as 'do not change the current value'.
When a frame_txtime of zero is required for testing purposes this
CAN_ISOTP_FRAME_TXTIME_ZERO u32 value has to be set in frame_txtime.

Link: https://lore.kernel.org/all/20220309120416.83514-2-socketcan@hartkopp.net
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

1 files changed, 11 insertions, 1 deletions

diff --git a/net/can/isotp.c b/net/can/isotp.c
index d59f1758ac9c..47404ba59981 100644
--- a/net/can/isotp.c
+++ b/net/can/isotp.c
@@ -140,6 +140,7 @@ struct isotp_sock {
 	struct can_isotp_options opt;
 	struct can_isotp_fc_options rxfc, txfc;
 	struct can_isotp_ll_options ll;
+	u32 frame_txtime;
 	u32 force_tx_stmin;
 	u32 force_rx_stmin;
 	u32 cfecho; /* consecutive frame echo tag */
@@ -360,7 +361,7 @@ static int isotp_rcv_fc(struct isotp_sock *so, struct canfd_frame *cf, int ae)
 
 		so->tx_gap = ktime_set(0, 0);
 		/* add transmission time for CAN frame N_As */
-		so->tx_gap = ktime_add_ns(so->tx_gap, so->opt.frame_txtime);
+		so->tx_gap = ktime_add_ns(so->tx_gap, so->frame_txtime);
 		/* add waiting time for consecutive frames N_Cs */
 		if (so->opt.flags & CAN_ISOTP_FORCE_TXSTMIN)
 			so->tx_gap = ktime_add_ns(so->tx_gap,
@@ -1293,6 +1294,14 @@ static int isotp_setsockopt_locked(struct socket *sock, int level, int optname,
 		/* no separate rx_ext_address is given => use ext_address */
 		if (!(so->opt.flags & CAN_ISOTP_RX_EXT_ADDR))
 			so->opt.rx_ext_address = so->opt.ext_address;
+
+		/* check for frame_txtime changes (0 => no changes) */
+		if (so->opt.frame_txtime) {
+			if (so->opt.frame_txtime == CAN_ISOTP_FRAME_TXTIME_ZERO)
+				so->frame_txtime = 0;
+			else
+				so->frame_txtime = so->opt.frame_txtime;
+		}
 		break;
 
 	case CAN_ISOTP_RECV_FC:
@@ -1498,6 +1507,7 @@ static int isotp_init(struct sock *sk)
 	so->opt.rxpad_content = CAN_ISOTP_DEFAULT_PAD_CONTENT;
 	so->opt.txpad_content = CAN_ISOTP_DEFAULT_PAD_CONTENT;
 	so->opt.frame_txtime = CAN_ISOTP_DEFAULT_FRAME_TXTIME;
+	so->frame_txtime = CAN_ISOTP_DEFAULT_FRAME_TXTIME;
 	so->rxfc.bs = CAN_ISOTP_DEFAULT_RECV_BS;
 	so->rxfc.stmin = CAN_ISOTP_DEFAULT_RECV_STMIN;
 	so->rxfc.wftmax = CAN_ISOTP_DEFAULT_RECV_WFTMAX;